Therefore, in handfree telephones provided with microphones and speakers mounted close to each other, a technique for realizing two-way simultaneous speaking without causing howling, generally involves the use of speech switching circuits and echo cancelers. Echo cancelers are usually comprised of digital hardware and software using a digital signal processor (DSP). The algorithms used in echo cancelers are already established and technically speaking realization or use of these algorithms is of no great difficulty. The problem, however, is the production costs in commercialization and the physical size of the hardware and software. The component accounting for the majority of the cost of the echo canceler is the digital signal processor. If the processing performance of digital signal processors were the same, then the cost of the DSP would be proportional to the echo continuation time to be canceled out. In other words, the cost and size of the DSP may be said to be determined by the echo cancelable time. Therefore, the proper setting of the echo cancelable time is important and has a decisive effect on the price competition at the time of marketing the handfree telephones.
However, the superiority or inferiority (echo canceling ability) of a handfree telephone with a built-in echo canceler is determined by the length of the set reverberation time when comparing echo cancelers with the same coefficient correcting algorithms. The longer the reverberation time, naturally the more the apparatus can be used in rooms with different and longer reverberation times and a higher or better performance of the handfree telephone.
Therefore, the policy in design tends to be to work to lengthen the set reverberation time as much as possible which substantially increases the cost and size of the echo canceler. For this reason, handfree telephones with built-in echo cancelers have become high in price and large in size and the ones in development or on the market have ended up limited for use in conference telephones or car telephones where high prices are tolerated by the consumer. Of course, the price cannot be set exorbitantly high and therefore in many cases a reverberation time of several tens of milliseconds (msec) to a hundred odd msec is selected to be used as the set echo cancelable time.
In addition, the set echo cancelable time in the echo canceler may be shortened so as to shorten the convergence time of the adaptive filter constituting the echo canceler.
The following conventional methods of shortening the convergence time of the adaptive filter are known:
(1) Japanese Unexamined Patent Publication (Kokai) No. 62-291223 "Adaptive Filter Adaptation Method and Apparatus" (laid open Dec. 18, 1987),
(2) Japanese Unexamined Patent Publication (Kokai) No. 63-248218 "Adaptive Control Filter" (laid open Oct. 14, 1988), and
(3) Japanese Unexamined Patent Publication (Kokai) No. 1-233826 "Adaptive Filter" (laid open Sep. 19, 1989).
The reference of the above-mentioned (1) No. 62-291223 discloses a method using a correlator to shorten the convergence time of the adaptive filter.
The reference of the above-mentioned (2) No. 63-248218 discloses a method of shortening the convergence time by providing a tap number determining circuit for controlling and changing the number of taps.
The reference of the above-mentioned (3) No. 1-233826 also discloses an adaptive filter which shortens the convergence time by changing the number of taps.
The problem, however, is that there is little evidence to determine if the convergence time of the adaptive filter is sufficient or insufficient or excessive and to determine if the convergence time is deficient with the time setting method of the prior art. The echo cancelable time in handfree telephones previously announced seem to have been already judged on the basis of "the longer the better, but to an extent considering the rise in price". Judgements based on such scanty evidence lead to the problems of rising prices when the convergence time setting is excessive and unnecessary and the occurrence of howling when the time set is too short.
Regarding the reduction of the amount of arithmetic operations in the echo canceler, there are known the following methods:
(4) "A Study on Echo Cancelers Using the Learning Identification Method" (B-2-3, 4th Digital Signal-Processing Symposium (December 1989)), and
(5) "Thoughts on Echo Cancelers Using the Learning Identification Method" (3-6-6, Fiscal 1989 Japan Acoustical Society Autumn National Congress (October 1989)).
Further, the fact that the echo path gain displays an attenuation characteristic is shown in the following articles:
(6) Institute of Electronics, Information and Communication Engineers Technical Reports EA89-3 "Regarding Improvement of Adaptive Characteristic of Echo Cancelers in Room Sound Fields", Makino et al., and
(7) Institute of Electronics Information and Communication Engineers Spring National Congress B-557 "Examination of Echo Cancelers Using Weighted Adaptive Control", Maruyama.
Further, prior art on the detection of a narrow band signal by the linear prediction method is described in the following articles:
(8) "Examination of Single Tone Detectors Using Adaptive Filters" (1990 Institute of Electronics, Information and Communication Engineers Spring National Congress B-738),
(9) "Examination of Narrow Band Signal Detection Method in Echo Canceler Type Handfree Circuit" (1988 Institute of Electronics, Information and Communication Engineers Autumn National Congress B300), and
(10) "Narrow Band Signal Detection Apparatus", Japanese Examined Patent Publication (Kokoku) No. 2-65325.
Further, even if the echo cancellation time is suitably set, the handfree telephone components should desirably be made on one chip since the handfree telephone components are carried in the handfree telephone. On the other hand, the size of the echo canceler apparatus becomes larger along with the amount of arithmetic operations handled when increasing the set reverberation time. Therefore, to make the echo canceler fit on one chip, it is desirable to reduce the amount of arithmetic operations handled.
Also, even if the echo cancelable time is suitably set and the amount of arithmetic operations in the echo canceler is slashed or reduced, the coefficient estimation operation of the echo canceler based on the narrow band signal of the ring back tone, time information, etc. input to the handfree telephone results in the estimated narrow band signal deviating from the narrow band signal of the next input speech signal. The amount of echo cancellation with respect to the input of the speech signal directly after the distorted or incorrectly estimated narrow band signal is incorrect and howling occurs. In addition, a distorted narrow band signal is output from the speaker, which makes commercialization of the handfree telephone impossible. This problem has been dealt with or handled in the past by detecting the distorted narrow band signal and removing the unwanted or distorted signal. However, there are problems such as the possibility of incorrectly determining female voices, etc., as a narrow band signal, the long delay time occurring in the detection of a narrow band signal, and the erroneous identification of howling as a narrow band signal.